The effects of face spatial frequencies on cortical processing revealed by magnetoencephalography

To study the spatial frequency (SF) effects on cortical face processing, we recorded magnetoencephalographic responses in seven healthy subjects to upright and inverted human faces. Four face types were used, including original (broad-band SF, BSF), low SF (LSF, <5 cycles/face), middle SF (MSF, 5-15 cycles/face), and high SF (HSF, >15 cycles/face) face images. Using equivalent current dipole (ECD) modeling, neuromagnetic M170 responses peaking around 160-185 ms were localized in right occipitotemporal region across subjects to BSF faces. M170 responses to LSF faces showed longer latency and smaller amplitude compared with those to BSF faces. We found no significant difference between BSF, MSF, and HSF conditions in M170 amplitude or latency. ECD locations for the four upright face conditions were close to one another, although the mean locations for MSF or HSF seemed more medial than those for BSF or LSF. Longer latencies for inverted than upright faces were observed in BSF (183.4 ± 8.5 ms versus 168 ± 6.9 ms, P < 0.001) and LSF face conditions (223.6 ± 13.1 ms versus 207.3 ± 16.3 ms, P < 0.01). M170 ECDs were located more medial for inverted than upright images in either BSF or LSF condition. In conclusion, the less M170 activation to LSF faces suggests that face parts information is important for early face processing. The cortical representations in right occipitotemporal region for configural and face feature processing are overlapping. Our findings on the face inversion effect suggest that inverted BSF and LSF faces may be processed as objects.